The ball ramp assembly comprises an axially supported disc and a disc which is axially displaceable relative thereto. On their end surfaces facing one another, the two discs comprise a plurality of circumferentially distributed ball grooves which, around their respective circumference, comprise a depth which is variable in opposite directions. Each two ball grooves positioned opposite one another form a pair and in each pair of ball grooves, a ball is held by which the discs axially support one another. The two discs can be rotated relative to one another by a motor, with a rotation effecting an axial displacement of the axially displaceable disc relative to the axially supported disc, and thus an actuation of the multi-plate coupling.
Axial setting devices of this type for actuating multi-plate couplings are widely used in the drivelines of motor vehicles. The multi-plate couplings can be used as locking couplings in differential drives, or as so-called hang-on couplings for optionally driving an additional driving axle, or as friction couplings for achieving a variable torque distribution between two driving axles of a differential drive.
From U.S. Pat. Nos. 4,950,214 and 5,106,349, there are known axial setting devices of said type for actuating locking couplings in differential drives. In order to achieve a rapid reaction behavior in a first actuation phase and accurate coupling control in a second actuation phase, the ball grooves comprise non-linear control curves having initially a steeper, and then, a progressively flattening profile. Thus, in the first actuation phase, there is achieved a high rate of spread between the discs, and in the second actuation phase there is achieved a low rate of spread.
U.S. Pat. No. 6,571,928 discloses a two-stage ball ramp assembly with a setting disc and a supporting disc which are rotatable relative to one another. The supporting disc, on its side facing away from the setting disc, comprises a plurality of ramps which cooperate with ramps which are provided at the housing and whose depth varies around the circumference in opposite directions. Furthermore, the supporting disc, in its surface facing the setting disc, comprises ball grooves with a circumferentially variable depth. The angle of the ramps is steeper than the angle of the ball grooves.
U.S. Pat. No. 7,000,492 discloses a ball ramp assembly which provides spring support during the return movement. In the region adjoining an end position defined by the greatest groove depth, the ball grooves comprise a rising run-out which, when the two discs are rotated beyond the end position, cause the discs to move away from one another.
Especially in differential assemblies provided for a variable torque distribution, as known from DE 103 42 164 A1 for example, there is required a rapid and accurate friction coupling control by means of suitable axial setting devices in order to ensure the driving stability of the motor vehicle. Thus, there exists a need for an improved ball ramp assembly with a quick reaction time.